When color image data is to be transmitted or accumulated, the color image data is subjected to the processing for compression and coding in order to suppress an amount of information to be processed.
A conventional type of image data compression method is generally a method, as typified by JPEG (Joint Photographic Expert Group) method, of dividing image data into blocks each having a plurality of pixels, subjecting each of the blocks to specified conversion and then compressing the data by quantization. A technology related to the color image data converting apparatus is disclosed in Japanese Patent Laid-Open Publication No. SHO 63-185163.
In the color image data converting apparatus disclosed in this Publication, as shown in FIG. 7, at first, image data for inputted three colors of RGB is converted into data for lightness (Y) and chromaticity (u, v) in a color space converting section 510. Then, each of the data (Y, u, v) is inputted into corresponding space converting sections 521, 522 and 523, where it is subjected to space conversion for each block having a plurality of pixels. Then, the data is quantized in corresponding quantizing sections 531, 532 and 533 and coded in a coding section 540. By converting an input color image data into information for lightness and color, and then subjecting the data to space conversion, quantization and coding, compression efficiency of color image data can be enhanced.
In the conventional type of technology as disclosure in the above Publication, however, the data for lightness (Y) and chromaticity (u, v) converted in the color space converting section 510 is uniformly quantized in each of the quantizing sections 531, 532 and 533 regardless of characteristics of input color image data such as a difference in its lightness or the like. Therefore, the compression may be insufficient or it may be too heavy depending on color, and hence image quality may be degraded.